The present invention generally relates to a magnetic recording medium for a magnetic disk drive unit. More particularly, the present invention relates to the composition and positioning of a magnetic layer, a soft magnetic layer and control layers on a magnetic recording medium.
As data processing technology advances, magnetic disk drive units used as external storage devices for computers require larger capacities. For this reason, magnetic disks or recording media also must be capable of high density recording. High density recording has the benefit of producing high S/N ratios.
In in-plane magnetic recording media, it has become known that a reading waveform pulse width of Pw 50 creates the following relationships between the coercivity strength (Hc), saturation remanent magnetization (Mr), and the thickness of the magnetic layer (t), which are the magnetostatic properties of a recording medium:
axe2x88x9d(txc3x97Mr/Hc)xc2xdxe2x80x83xe2x80x83(1)
Pw 50=(2(a+d)2+(a/2)2)xc2xdxe2x80x83xe2x80x83(2)
Where d represents the magnetic spacing.
In simple terms, the narrower the pulse width, the better the medium is suited for high density recording. Therefore, to provide efficient high density recording, the smaller xe2x80x9caxe2x80x9d is in the above equation, the better. To make xe2x80x9caxe2x80x9d smaller, t and Mr must be as small as possible, and the coercivity (Hc) must be as large as possible.
Since reducing t and Mr of the magnetic layer also leads to a reduction in noise, demands for higher recording densities for magnetic recording medium have consistently been accompanied by demands to reduce t and Mr. However, as the t and Mr of the medium become smaller, thermal decay of magnetization occurs more easily, which creates a phenomenon where the signal level drops as time passes.
In order to prevent thermal decay, U.S. Pat. No. 5,041,922 to Wood et al. discloses a magnetic recording medium that uses a soft magnetic layer that possesses high magnetic permeability and is called a keeper layer. In this known medium, the soft magnetic or keeper layer is positioned either directly above or directly below the magnetic layer. Both of these layers rest on a non-magnetic substrate layer.
This configuration causes a mirror image of the bit pattern recorded on the magnetic layer to be formed on the keeper layer. The reverse magnetic field of the bit boundary will then be lowered by the recording condition, which stabilizes the magnetization. Stabilizing the magnetization also provides high resistance to thermal decay. A known keeper layer is also disclosed in U.S. Pat. Nos. 5,830,590 and 5,870,260. These patents disclose that signals are read by using magnetic heads to create a bias magnetic field and then saturating the keeper layer.
However, in these known magnetic recording medium with keeper layers, the S/N ratio is unacceptable when the keeper layer is placed above the magnetic layer because the distance between a magnetic head traveling above the medium and the magnetic layer is too far. In addition, when the keeper layer is placed below the magnetic layer, the in-plane orientation of the magnetic layer""s C axis deteriorate so that the resolution is reduced when reading, which increases the noise. Moreover, known MR or GMR heads used for reproduction generate a magnetic field by using very small sensing currents that cannot adequately saturate the keeper layer. This also will make it very difficult to read signals.
For these reasons, a main object of the present invention is to provide an improved magnetic recording medium that is suitable for high density recording.
Another object of the present invention is to provide an improved magnetic recording medium with a high S/N ratio.
Yet another object of the present invention is to provide an improved magnetic recording medium with good in-plane orientation of the magnetic layer.
Still another object of the present invention is to provide a magnetic recording medium in which reading is possible using either a MR head or a GMR head.
Other objects or advantages of the present invention are discussed or will be apparent from the detailed description of the invention.
In one aspect of the present invention, a magnetic recording medium includes a non-magnetic substrate, a first non-magnetic control layer disposed above the substrate, a soft magnetic layer disposed above the first control layer, and a second non-magnetic control layer disposed above the soft magnetic layer. A primary magnetic layer is disposed above the second control layer.